1. Field of the Invention
This invention relates to a method for preparing rare earth nitride magnets which find main use in motors as resin bonded magnets.
2. Background of the Invention
Among high performance rare earth magnets, Sm--Co magnets and Nd--Fe--B magnets have been used in practice while active research efforts are now made to develop novel rare earth magnets.
For example, there were proposed rare earth nitride magnets of the Sm--Fe--N system in which nitrogen forms an interstitial solid solution with Sm.sub.2 Fe.sub.17 crystals. It was reported in Paper No. S1.3 at the Sixth International Symposium on Magnetic Anisotropy and Coercivity in Rare Earth-Transition Metal Alloys, Pittsburgh, Pa., Oct. 25, 1990 (Proceedings Book: Carnegie Mellon University, Mellon Institute, Pittsburgh, Pa. 15213, USA) that the basic physical properties of 4.pi.Is=15.4 kG, Tc=470.degree. C. and H.sub.A =14 T are achievable with approximate compositions of Sm.sub.2 Fe.sub.17 N.sub.2.3, that metal bonded magnets using zinc as a binder exhibit a (BH)max of 10.5 MGOe, and that the introduction of nitrogen into Sm.sub.2 Fe .sub.17 intermetallic compounds achieves a substantial improvement in Curie temperature and hence, in thermal stability.
A variety of proposals have been made on rare earth nitride magnets (to be referred to as Sm--Fe--N magnets, hereinafter) because they are theoretically expected to exhibit characteristics surpassing Nd--Fe--B magnets. For the enhanced performance, especially high magnetization of Sm--Fe--N magnets, it is effective to increase the content of an .alpha.-Fe phase in the magnet. The content of an .alpha.-Fe phase may be increased by reducing the amount of rare earth elements in the entire magnet, and reducing the amount of rare earth elements used, in turn, has the advantage of reduced costs. However, simply reducing the amount of rare earth elements to increase the .alpha.-Fe phase detracts from coercivity, rather resulting in poorer magnet properties. Then the following proposals have been made.
(1) We proposed in U.S. Ser. No. 08/500,578 a Sm--Fe--N magnet consisting essentially of 4 to 8 at % of R, 10 to 20 at % of nitrogen, 2 to 10 at % of M, and the balance of T wherein R is at least one rare earth element, samarium accounting for the majority, M is an additive element containing zirconium as an essential component, and T is a transition metal such as iron, the magnet comprising a hard magnetic phase of TbCu.sub.7 type and a soft magnetic phase consisting of a bcc structured T phase such as an .alpha.-Fe phase, the soft magnetic phase having a mean grain size of 5 to 60 nm and being present in a proportion of 10 to 60% by volume. This magnet is characterized by the essential inclusion of zirconium, the limited mean grain size of the soft magnetic phase, and the limited proportion of the soft magnetic phase in the magnet. Owing to these restrictions, a relatively high coercivity is available although good magnetization is achieved by reducing the content of rare earth elements to 8 at % or less.
(2) JP-A 81741/1996 discloses a magnet material having a composition represented by R.sup.1.sub.x R.sup.2.sub.y T.sub.100-x-y-z-v M.sub.z N.sub.v wherein R.sup.1 is at least one rare earth element, R.sup.2 is at least one element of zirconium, hafnium and scandium, T is at least one element of iron and cobalt, M is at least one element of Ti, V, Nb, Ta, Cr, Mo, W, Mn, Ni, Ru, Rh, Pd, Cu, Ag, Zn, Cd, Al, Ga, In, Si, Ge, Sn, and Sb, letters x, y, z and v are 2.gtoreq.x.gtoreq.20, 0.gtoreq.y.gtoreq.15, 2.gtoreq.x+y .gtoreq.20, 0.gtoreq.z.gtoreq.20, and 0.01.gtoreq.v.gtoreq.20, expressed in at %, the magnet material having a phase having a TbCu.sub.7 type crystalline structure as the major phase containing at least 90 at % of the element T. It is alleged that the saturation magnetic flux density of the major phase can be improved by containing at least 90 at % of the element T in the major phase. With respect to the .alpha.-Fe phase, it is intended to prevent the precipitation of the .alpha.-Fe phase.
The magnets of (1) exhibit properties surpassing the Nd--Fe--B magnets which have been used in the industry, but further improvements in coercivity and squareness ratio to be discussed later are desirable. The properties of the magnets of (2) are insufficient for use in spindle motors for computer hard disk drives.